1. Technical Field
This invention describes a biocomposite material, which is especially suitable for bone surgical applications.
2. Background Art
It has been found that many ceramic materials have properties, which allow their use as bone graft materials. Ceramic materials (bioceramics), which are tissue compatible and/or which form chemical bonds with bone tissue and/or which promote the growth of bone tissue, are e.g. calcium phosphate: apatites such as hydroxyapatite, HA, Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2 (R. E. Luedemann et al., Second World Congress on Biomaterials (SWCB), Washington, D.C., 1984, p. 224), trade names such as Durapatite, Calcitite, Alveograf and Permagraft; fluoroapatites; tricalciumphosphates (TCP) (e.g. trade name Synthograft) and dicalciumphosphates (DCP); magnesium calcium phosphates, .beta.-TCMP (A. Ruggeri et al., Europ. Congr. on Biomaterials (ECB), Bologna, Italy, 1986, Abstracts, p. 86); mixtures of HA and TCP (E. Gruendel et al., ECB, Bologna, Italy, 1986, Abstracts, p. 5, p. 32); aluminum oxide ceramics; bioglasses such as SiO.sub.2 -CaO-Na.sub.2 O-P.sub.2 O.sub.5, e.g. Bioglass 45S (structure: SiO.sub.2 45 wt %. CaO 24.5%, Na.sub.2 O 24,5% and P.sub.2 O.sub.5 6%) (C. S. Kucheria et al., SWBC, Washington, D.C., 1984, p. 214) and glass ceramics with apatites, e.g. MgO 4,6 wt %, CaO 44,9%, SiO.sub.2 34,2%, P.sub.2 O.sub.5 16,3% and CaF 0,5% (T. Kokubo et al., SWBC, Washington, D.C., 1984, p. 351) and calcium carbonate (F. Souyris et al., EBC, Bologna, Italy, 1986, Abstracts, p. 41).
The applications of the above ceramic materials as synthetic bone grafts have been studies by different means by using them for example both as porous and dense powder materials and as porous and dense macroscopical samples as bone grafts. Also ceramic powder-polymer composites have been studied in this way (e.g. W. Bonfield et al.. SWBC, Washington, D.C., 1984, p. 77).
Some bioceramics are resorbable like for example tricalciumphosphate (see e.g. P. S. Eggli et al., ECB, Bologna, Italy, 1986, p. 4) and calcium carbonate (F. Souyris et al., ibid, p. 41). The most known of the nonresorbable bioceramics is aluminum oxide. In literature it has been reported that some bioceramics, like hydroxyapatite, are both resorbable (W. Wagner et al., ECB, Bologna, Italy, 1986, Abstracts, p. 48) and nonresorbable (biostable) (e.g. G. Muratori, ibid, p. 64). Resorbable bioceramics dissolve in tissues slowly and/or they are replaced by the minerals of bone tissue. On the other hand, the biostable bioceramics remain in the tissues in an unchanged state, in such a way that the bone tissue grows into contact with the bioceramic.
Porosity of the bioceramic is advantageous, because the bone tissue can grow into the open porosity, if the pores have a suitable size. On the other hand, a problem of macroscopic bioceramic samples and especially of porous samples is their brittleness. It has been tried to compensate for the brittleness of bioceramics by manufacturing of ceramic powders and of biostable or of resorbable polymers composites, where the ceramic powder particles have been bound together by means of a polymer. This has been achieved e.g. by pressing the mixture of bioceramic powder and polymer powder by means of heat and pressure into a composite piece or by binding bioceramic powder by means of a reactive polymer to a composite piece. Such composites are tough when suitable polymers are applied. Composites of a bioceramic powder and resorbable polymer have been described e.g. in Finnish patent application 863573.
The ceramic powder-polymer composites have a disadvantage that the presence of binding polymeric material prevents the direct contact of bioceramic powder particles and bone tissue to each other, and therefore delays and prevents the growth of the bone tissue on the surface of composite material and inside of it, because the bone tissue does not have such an affinity to grow on the surface of biostable or resorbable organic polymers as it has to grow on the surface of bioceramics or into their internal open porosity. As a consequence the growth of new bone and the healing of tissue proceeds more slowly with bioceramic-polymer composites than with pure bioceramics (e.g. according to S. Ishida et al., ECB, Bologna, Italy, 1986, Abstracts, p. 86), the growth of new bone on the surface of 70% hydroxyapatite filler-triethyleneglycoldimethacrylate composite occurred in studies done with rabbits 2-3 times more slowly than the growth of new bone on the surface of pure sintered hydroxyapatite).
Bioceramics like hydroxyapatite are applied generally as bone graft materials in powder form for filling of bone defects or for alveolar ridge reconstruction by injecting the hydroxyapatite powder/water (or blood) mixture (particle size typically 10-50 mesh) on the bony surface of alveolar ridge into a cavity which has been done below the gingival tissue. The bone tissue grows rapidly into contact directly with hydroxyapatite particles, which when biostable remain as part of the forming new bone or are resorbed and replaced later with new bone.
The powder-like bone graft materials have, however, a disadvantage that they remain at their place only after the connective tissue and/or growing bone tissue binds them to their place. For example, in the case of hydroxyapatite powders applied for alveolar ridge augmentation this will take about one month. Before the powder particles have been bound to their place by means of tissue growth, the powder can move easily from the place, where it should be, when mechanical forces (e.g. biting) act upon the soft tissues which surround the powder particles. This can lead to a deterioration of good operation result or it is not achieved at all or it is achieved only partially.
The Finnish patent application 863473 and the corresponding PCT application FI87/00119 describe supporting structures which have been manufactured of resorbable polymer or composite and which can e applied to immobilize bioceramic particles to their place on the surface of bone. The applications of above invention have been restricted, however, surgically to such operations, where the bioceramic powder can be located on a certain restricted area which is surrounded by suitable tissues. The resorbable supporting structures of the above invention, like chute-like, box-like, flat tube or bag-like structures cannot be applied e.g. in the reconstructive surgery of flat bones of maxillofacial region or of skull. Additionally, the strength of the system comprising e.g. the chute or the corresponding and bioceramic powder is based only on the structure of the chute or the corresponding, when the bioceramic particles are not bound together by primary chemical bounds.
It has been found that bone tissue grows as a rule rapidly and without problems into bioceramic pieces which contain suitable open porosity. Because of the brittleness of these materials they can be, however, broken easily during operation or after it before the bone tissue has grown into the pore structure of ceramic material Also solid dense bioceramics are often brittle especially if they are thin, plate-like or curved pieces If the plate is broken during operation or soon after it the pieces of the plate can move in tissues and cause problems to the patient.
The strength, such as compression strength, of porous bioceramic pieces can be increased by coating the bioceramic sample with a resorbable or biostable polymer. The polymeric coating gives to the sample, however, only a limited increase of strength, because the strength of polymers is as a rule only moderate and the stiffness (the elastic modulus) is small in comparison to ceramics. Therefore, even small mechanical stresses can easily break the bioceramic part of composites manufactured of a porous bioceramic material and of a polymeric coating, because as a consequence of the small elastic modulus of polymer the external stresses are shifted already after small deformations to the ceramic component of the material.
EP-patent application 171884 describes an endoprosthetic device which comprises polyaryletherketone (PEEK) and possible additional biostable reinforcing fibres, such as carbon fibres, where the shifting of fibres into tissues can be prevented by means of a polymer-rich surface layer on the surface of the device. Further the above invention describes devices, which comprise a massive ceramic core component, which has been coated at least partially with tissue compatible biostable polymer. When the above devices are applied surgically, the biostable polymers and fibres remain permanently into the tissues of the patient and e.g. in the case of the application of ceramic block coated with a biostable polymer the polymer layer separates the ceramic core and the surrounding tissues permanently from each other preventing in this way the advantageous growth of surrounding tissues into direct contact with the ceramic block. In addition biostable polymers and fibres may release into surrounding tissues small particles, fibre fragments, or other debris as a consequence of wear, breakage or processing (see e.g. EP 171884, p. 14, lines 16-22). Small particles, fragments or debris cause as a rule in the surrounding tissues or in the nearby lymph nodes longlasting, even years lasting, foreign body and inflammation reaction.